In the January/February 2009 issue of PHOTO Techniques, Dick Dickerson and Silvia Zawadzky correctly said and showed that printing with two contrast filters— a low-contrast one such as Ilford’s #00 and a high-contrast one such as Ilford’s #5—will not give us “richer” black-and-white prints than printing with any one of the contrast f ilters provided by Ilford, Kodak, and other manufacturers, or with no filter.
They then concluded, not altogether correctly, that split-filter printing “does not afford access to a print appearance (contrast, curve shape) unattainable with single filters; it just takes longer to get there.”
In saying this, they overlooked one limitation of the manufacturers’ filter sets. The filters are designed to produce 12 distinct degrees of contrast, spaced half a contrast grade apart, from softest (lowest contrast) to hardest (highest contrast). The results per filter vary somewhat when we use different papers and/or different paper developers, but with any such combination we are locked into those 12 degrees of contrast when we use those f ilter sets, or the grade numbers furnished by enlargers with built-in filtration, one filter or setting at a time.
Incidentally, I have tested contrast filter sets sensitometrically, and have never found one that gave consistent or accurate half-grade differences from one filter to the next. The Kodak and Ilford sets that I’ve used are def initely quite uneven. Don’t worry about this; there’s nothing we can do about it. Also, gelatin contrast filters, which I use above the lens in my old but good Omega D2V enlarger, fade over time, as I learned one day when I saw that my #31/2 filter was giving me higher contrast than my #41/2, which had faded more. It was time to get a new set.